It is known to provide a roll control system for a motor vehicle which comprises a torsion bar which is formed in two parts, with an actuator positioned between the two parts. The actuator is controllably actuated to either allow free relative rotational movement of the two parts of the torsion bar about their longitudinal axes (for example, during straight line motion of the motor vehicle), or to create a torque between (or lock) the two parts together (for example, during cornering of the motor vehicle). Known roll control actuators, as, for example, described in GB-A-2212770 and GB-A-2220625 are complicated.